1. Field of the Invention
The present invention relates to a plant control system.
2. Description of the Prior Art
The applicant of the present application has proposed an air-fuel ratio control system for controlling the air-fuel ratio of an internal combustion engine (more precisely, the air-fuel ratio of a mixture of fuel and air to be combusted in the internal combustion engine) for enabling an exhaust gas purifier such as a catalytic converter, e.g., a three-way catalytic converter, disposed in the exhaust passage of the internal combustion engine to have an optimum purifying capability. See, for example, Japanese laid-open patent publication No. 11-324767 and U.S. patent application Ser. No. 09/311,353 for details.
According to the proposed air-fuel ratio control system, an exhaust gas sensor for detecting the concentration of a certain component of an exhaust gas that has passed through the exhaust gas purifier, such as an O.sub.2 sensor (oxygen concentration sensor), is disposed downstream of the exhaust gas purifier, and the air-fuel ratio of the internal combustion engine is manipulated to converge an output of the O.sub.2 sensor, i.e., the detected value of the oxygen concentration, to a predetermined target value (constant value) for enabling the catalytic converter to have an optimum purifying ability irrespective of the aging of the catalytic converter.
More specifically, an exhaust gas sensor (hereinafter referred to as "air-fuel ratio sensor") disposed upstream of the exhaust gas purifier for detecting the air-fuel ratio of an air-fuel mixture combusted in the internal combustion engine and an exhaust system ranging from the air-fuel ratio sensor to the O.sub.2 sensor and including the exhaust gas purifier is regarded as a controlled object. The air-fuel ratio control system has a control processing means for sequentially generating a target air-fuel ratio for the internal combustion engine (an air-fuel ratio for the internal combustion engine required to converge the output signal of the O.sub.2 sensor toward a target value), which serves as a control input to the controlled object, and an air-fuel ratio manipulating means for adjusting the amount of fuel supplied to the internal combustion engine in order to converge the output signal of the air-fuel ratio sensor (the detected value of the air-fuel ratio) to the target air-fuel ratio generated by the control processing means for thereby manipulating the air-fuel ratio of the internal combustion engine into the target air-fuel ratio.
Generally, the catalytic converter as the exhaust gas purifier generally has a relatively long dead time. When the rotational speed of the internal combustion engine is low, e.g., when the internal combustion engine is idling, the internal combustion engine and the air-fuel ratio manipulating means also have a relatively long dead time. These dead times present an obstacle to the process of stably controlling the output signal of the O.sub.2 sensor at the given target value.
The above air-fuel ratio control system has an estimating means for sequentially generating an estimated value of the output signal from the O.sub.2 sensor after a total dead time which is the sum of the dead time (which is required for the air-fuel ratio detected at each time by the air-fuel ratio sensor to be reflected in the output signal from the O.sub.2 sensor) of the exhaust system including the exhaust gas purifier and the dead time (which is required for the target air-fuel ratio generated at each time by the control processing means to be reflected in the actual air-fuel ratio) of a system that comprises the internal combustion engine and the air-fuel ratio manipulating means. The control processing means sequentially generates the target air-fuel ratio for converging the estimated value generated by the estimating means to the target value for the output signal from the O.sub.2 sensor, for thereby compensating for the effect of the dead time. The response delay of the internal combustion engine of the system that comprises the internal combustion engine and the air-fuel ratio manipulating means can be compensated for by the air-fuel ratio manipulating means. Accordingly, the estimated value for the output signal from the O.sub.2 sensor after the total dead time is determined using the output signals from the air-fuel ratio sensor and the O.sub.2 sensor according to an algorithm constructed based on a model representing the behavior of the exhaust system including the response delay of the exhaust system.
In the above air-fuel ratio control system, the exhaust system including the exhaust gas purifier can be considered as a plant for generating the output signal of the O.sub.2 sensor from the air-fuel ratio of the internal combustion engine as detected by the air-fuel ratio sensor (the output signal from the air-fuel ratio sensor). The internal combustion engine can be regarded as an actuator for generating an exhaust gas having an air-fuel ratio as an input to the plant. The air-fuel ratio sensor and the O.sub.2 sensor can be regarded as detecting means for detecting the input and output, respectively, of the plant.
For making the exhaust gas from the internal combustion engine cleaner, the catalytic converter as the exhaust gas purifier may be increased in size or the exhaust gas purifier may comprise a plurality of joined catalytic converters.
In such a system, however, since the exhaust system including the exhaust gas purifier as the object to be controlled by the control processing means has a long dead time and the behavior of the exhaust system tends to be complex, it is difficult for the estimated value for the output signal from the O.sub.2 sensor as generated by the estimating means to have a sufficient level of accuracy. Therefore, the stability and quick response of the control process for converging the output of the O.sub.2 sensor to the target value is reduced, and the catalytic converter between the air-fuel ratio sensor and the O.sub.2 sensor fails to perform a sufficient exhaust gas purifying capability.
The above drawbacks are not limited to the exhaust system including the exhaust gas purifier, but occurs in a system that controls the operation of an actuator to control the output of a plant, which is generated from an input generated by the actuator, at a certain target value, if the plant has a long dead time.